The present invention is driven by the global terrorism threat. As the antiterrorism situation getting severe, the radiographic container inspection systems capable of automatically detecting explosive, drugs and other contraband become pressing desired. The existing material discrimination techniques for container and other voluminous objects inspection, such as: high and dual energy radiographic technique, PFNA and container inspection CT, show more and more important.
High and dual energy radiographic technique employs absorption variation between materials in the megavolt range due to the Compton Effect and Pair Production Effect to determine the effective atomic number of the irradiated objects, and accordingly to discriminate different materials. But there are some physical limitations: First, absorption variation is not big enough. Second, the high-energy spectrum partially overlapped with low energy spectrum, even with spectrum filtering only can solve part of problems. Third, measurement error degrades the discrimination effect. All this factors make the unsatisfied results, and sequentially high and dual energy system mainly used to identify “organic”, “compound” and “inorganic” material in inspected container. Isotope source can provide monoenergetic gamma-ray which can solve the spectrum overlap problem, but penetration ability is too low to be used in container and other voluminous objects inspection system for material discrimination.
Present available PFNA systems, some have 3D material discriminating ability. But their spatial resolution is too big, throughput is too slow, and the price is too high. So PFNA can't dominate container inspection market at present and near future. Some NAA container inspection system using Cf-252 as neutron source can't be used for on-line real-time measurement, because only after the suspicious area having been detected by other equipment, the NAA measurement for suspicious area can be performed.
Container inspection CT system is giant and the throughput rate is too low to dominate container inspection market.
WO 2004/053472 discloses a radiographic equipment which directly measure transmitted monoenergetic fast neutron and monoenergetic gamma-ray. This equipment employs mass attenuation coefficient ratio to discriminate different materials, which can be used for detecting explosives, drugs and contraband. Comparing with high and dual energy X-ray technique, the direct measuring transmitted dual-ray technique has better material identification ability. Comparing with PFNA technique which measures secondary radiations such as neutron-induced gamma rays, the direct measuring transmitted dual-ray technique has much more efficient, especially has high penetration ability than thermal neutron. Comparing with container inspection CT, the dual-ray system is compact, low price and real-time measurement.
Unfortunately, the monoenergetic dual-ray system only can use isotope source, such as Co-60, as its gamma-ray source. Nevertheless, for containers or other voluminous objects inspection, the big disadvantage of isotope source is low penetration ability, low spatial resolution, poor image quality, and has radiation safety administration problems. This technique only provides low spatial resolution container transmission image, so is difficult to compete with Linac container inspection systems which provide high quality image. Since monoenergetic gamma-ray has low penetration ability, which limits its material identifying thickness as well, can't be used in case of fully-loaded container or thick objects inspection. All those defects limit its applications.